"Because the MCS map field [of the VHT Capabilities Information Element] only allows three options (MCS 0 through 7, MCS 0 through 8, and MCS 0 through 9), it is not possible to disallow low data rates in an 11ac network."

Only two bits are used to define the 3 possibilities ( 0, 1, or 2), and the value of 3 is reserved.

My original confusion over this was that the Supported MCS Set map in an OmniPeek trace seems to imply something else, and that on some Cisco AP's, the GUI interface implies that you can make other choices - but doesn't allow them.

In a Motorola AP CLI manual, the available options only give you the three choices, but does not explain why. In fact, because it's not explained, it can mislead you to believe that the manual is missing parameters or keywords.